1. Field of the Invention
The present invention relates to an improved method of producing corrugated cardboads, and encompasses an apparatus for practicing the method.
2. State of the Art
As well known, corrugated cardboards are produced by the steps of adhering a sheet of corrugated paper with a sheet of liner to form a half-lined corrugated paper, and then, adhering another sheet of liner thereto. In some cases, two or more sheets of the half-lined corrugated paper are used to produce a "double-backer" or thicker ones.
As far as adhesive effect is concerned, synthetic resin-based adhesives such as emulsion of poly vinyl acetate are preferable. However, from the view to reduce the producing cost of corrugated cardboard, efforts has been made to use less expensive pasty materials. One of the successful ways is to use raw or uncoocked starch, i.e. water suspension of starch particles (hereinafter referred to as "raw starch"). The raw starch is coated on the corrugated paper, and then a liner is pressed to the coated corrugated paper under heating, thereby to cause gellation of the starch and transform it to a paste. To apply heat under pressure, it is generally practiced that the liner is contacted and slided on arranged series of "hot plates", or metal plates which supply heat.
Arrangement of the hot plates requires considerable floor area for the corrugated cardboard producing machine. The hot plates consume much heat energy, and moreover, it is often experienced that product cardboards suffer from scratches and (or) warp. It has been proposed, with the intention of eliminating use of the hot plates, that the raw starch coated on the corrugated paper so as to cause gellation thereof prior to contacting with the liner (Japanese Patent Publication No. 24658/1972).
The method got good reputation as far as elimination of the hot plate is concerned. However, practical use revealed a disadvantage of low productivity due to limited line speed on machine. It has been generally practiced in gellation of raw starch to blow steam out of nozzles to heat the raw starch coated on the projecting portion of the corrugated paper. In order to increase line speed on machine, it is effective to provide sufficient heat to the raw starch so as to increase rate of gellation. Low-pressure (say, 0.2 kg/cm.sup.2 gauge) steam usually used has a temperature as low as 80.degree. to 85.degree. C. when blown out of the nozzles, and therefore, has insufficient thermal energy. To realize a higher heating temperature, we tried use of high-pressure steam. It was experienced that the blowing is too vigorous and sweeps off the coated raw starch, thus causing dissatisfactory adhesion. Supply of too much steam resulted in excessive moistening of the half-lined corrugated paper with water formed by condensation of the steam. Though the scratching and (or) warp caused by use of the hot plates are avoided, excessive moistening causes another problem of longer necessary time for drying the product and reduction of the increased line speed on machine. (If the corrugated cardboard still moistened is processed in carton-making step, product cartons will have inaccurate size and insufficient strength.)
Further, according to our experience, it will give better adhesion to incompletely gell the raw starch before application of pressure and to complete the gellation by additional heating at and after the application of pressure, then to completely gell the raw starch before the application of pressure as practiced in the above mentioned prior method. It is believed that this is due to better infiltration of the incompletely gelled starch into the structure of the mating liner. Thus, the technology disclosed in the above noted Japanese Patent Publication No. 24658/1972 could be practiced better with partial use of the conventional hot plates.